Terpolymers for use as high temperature fluid loss additive and rheology stabilizer for high pressure, high temperature oil well drilling fluids

ABSTRACT

Novel water-soluble terpolymers comprising: 
     
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     Ingredients           Mole %                                              
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2-acrylamido-2-methylpropane-                                             
                      58-77                                               
sulfonic acid, sodium salt (AMPS)                                         
N--vinylpyrrolidone   2-20                                                
acrylonitrile         4-36                                                
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     These terpolymers provide high temperature fluid loss additives and rheology stabilizers for high calcium-containing brine clay drilling fluids.

INTRODUCTION

A serious problem is encountered when clay-based oil well drillingfluids are subjected to conditions of high temperature and high pressurein conjunction with utilization of high calcium-containing brines whichare used to prepare these drilling fluids. When these conditions exist,conventional drilling fluid polymeric additives such as acrylamidepolymers, when used as stabilizers for these fluids, tend to be renderedineffective.

Recently a partial solution to the problem has been provided by theutilization as a fluid loss additive and rheology stabilizer ofterpolymer composed of:

    ______________________________________                                        Ingredients           Mole %                                                  ______________________________________                                        2-acrylamido-2-methylpropane-                                                                       52.0                                                    sulfonic acid, sodium salt (AMPS)                                             acrylamide            32.0                                                    N--methyl-N--vinyl acetamide                                                                        16.0                                                    ______________________________________                                    

This terpolymer, while proving to be effective as a high temperaturefluid loss additive for oil well drilling fluids, has the disadvantageof using in its composition 16.0 mole % of N-methyl-N-vinyl acetamide.This renders the material expensive to use.

If it were possible to use a polymeric high temperature fluid lossadditive and rheology stabilizer that was resistant to highcalcium-containing brines, an improvement in the art would be afforded.

THE INVENTION

The invention comprises the utilization of terpolymers with thefollowing composition:

    ______________________________________                                                          Mole %                                                      Ingredients         Preferred                                                                              General                                          ______________________________________                                        2-acrylamido-2-methylpropane-                                                                     58.9     58-77                                            sulfonic acid, sodium salt (AMPS)                                             N--vinylpyrrolidone 08.3     2-20                                             acrylonitrile       32.8     4-36                                             ______________________________________                                    

As will be shown hereafter, these terpolymers provide excellent hightemperature fluid loss protection to a clay-based brine prepareddrilling fluid.

These terpolymers may be prepared in accordance with the followingpreparative technique, illustrating the preparation of the preferredterpolymer listed above:

A polymerization vessel was fitted with a stirrer, thermometer,condenser and addition port. The vessel was charged with 325 g of water,53 g of AMPS, 21 g of a 50% solution of sodium hydroxide, 21 g of a 50%solution of sodium hydroxide, 4 g of N-vinylpyrrolidone and 7.6 g ofacrylonitrile. The pH was adjusted to 8.3 and the polymerization wasinitiated by the sequential addition of 10 ml. of a 0.5% solution ofsodium bisulfite and 10 ml. of a 0.5% solution of ammonium persulfate.After two hours reaction time, the temperature was raised to 75° C. and10 ml of a 2% solution of sodium bisulfite and 10 ml of a 2% solution ofammonium persulfate was added. The reaction was heated for one hourbefore cooling to room temperature. The polymer solution had aBrookfield viscosity of 2940 cps.

The amount of polymer used to stabilize clay-based drilling fluids underconditions of high temperature and pressure may vary between about 10%up to 60% by weight based on the clay present in the fluid.

The polymers may be further characterized as having a molecular weightbelow 1,000,000. Preferably, they have a molecular weight within therange of 10,000-500,000.

To illustrate the invention, the preferred composition shown above wastested in a high temperature, high pressure fluid loss additive testwhich is described in detail in the American Petroleum Institutepublication, RP13B.

The particular drilling fluid tested contained 15 pounds/barrel of apre-hydrated Bentonite drilling fluid clay and a 50% dilution of a brineknown as a Williston Basin brine. This brine contained a substantialamount of calcium.

The blank test produced a filtrate of 42 ml/7.5 min.

The preferred composition of the invention was added to provide 6pounds/barrel of active polymer. This provided an approximate dosage of40% by weight active polymer based on active clay. At the end of 30minutes, the fluid loss in the above described test was 22 ml.

Using the same preparative technique and test method for determiningfluid loss additive effectiveness, the following additional polymerswere prepared and tested:

    ______________________________________                                        Monomer Mole %        HTHP                                                    AMPS     NVP*        ACN**    Fluid Loss                                      ______________________________________                                        58.9     8.3         32.8     22 ml.                                          62.0     2.3         35.7     48 ml.                                          76.5     19.3         4.1     47 ml.                                          ______________________________________                                         *NVP = N--vinylpyrrolidone                                                    **ACN = acrylonitrile                                                    

We claim:
 1. A method of improving high temperature fluid loss andrheology stabilization of high calcium brine clay-containing oil welldrilling fluids which comprises adding thereto a stabilizing amount of awater-soluble terpolymer comprising:

    ______________________________________                                        Ingredients          Mole %                                                   ______________________________________                                        2-acrylamido-2-methylpropane-                                                                      58-77                                                    sulfonic acid, sodium salt                                                    N--vinylpyrrolidone  2-20                                                     acrylonitrile        4-36                                                     ______________________________________                                    

and said terpolymer being further characterized as having a molecularweight below 1,000,000.
 2. A method of improving high temperature fluidloss and rheology stabilization of high calcium brine clay-containingoil well drilling fluids which comprises adding thereto a stabilizingamount of a water-soluble terpolymer comprising:

    ______________________________________                                        Ingredients          Mole %                                                   ______________________________________                                        2-acrylamido-2-methylpropane-                                                                      58.9                                                     sulfonic acid, sodium salt                                                    N--vinylpyrrolidone  08.3                                                     acrylonitrile        32.8                                                     ______________________________________                                    

and said terpolymer being further characterized as having a molecularweight below 1,000,000.